gettext: fix static build failure

When static libraries are preferred we don't build dynamic libraries so
it fails on the install phase. So don't try to do it for those cases.
Fixes:
http://autobuild.buildroot.net/results/651537ae1909b039e0cfc5c104ee87f93a9e5792/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>